The present invention relates to a signal attenuator for use in the front end of an FM tuner circuit, and more particularly to a radio-frequency signal attenuator suitable for use in an automatic gain control (AGC) circuit.
Radio waves transmitted from FM (frequency modulation) broadcast stations are received by aerial antennas. The radio waves received by the antennas are of a very weak level ranging from several microvolts [.mu.V] to several millivolts [mV]. Therefore, it is necessary to amplify the radio-frequency (RF) signal that is received by an FM receiver. In order to amplify an RF signal stably and sufficiently, it is first converted into a lower-frequency signal, i.e., a signal having an intermediate frequency (IF), which is internationally established as 10.7 [MHz]. The portion of an FM receiver which produces an IF signal is referred to as a "front end".
The front end of an FM receiver comprises an RF amplifier connected to the input terminal for amplifying a received RF signal, a local oscillator for generating a frequency (local oscillation frequency) which is necessary to produce an IF signal, and a mixer for mixing the local oscillation frequency and the RF signal thereby to generate an IF signal. There are known various front end designs which greatly govern the performance of FM receivers. According to one arrangement, the front end includes a tuner and an AGC circuit for receiving a desired broadcast RF signal with a good S/N ratio.
The AGC circuit extracts a portion of the IF signal from a front stage of an IF amplifier which is connected to the mixer, and produces, with a diode, a DC voltage that is proportional to the amplitude of the IF signal. Then, the DC voltage (AGC voltage) is fed back to the input stage of the front end, so that the attenuation of the input RF signal applied to the RF amplifier is regulated depending on the magnitude of the received RF signal. A signal attenuator is used to attenuate the input RF signal.
One conventional signal attenuator includes a PIN diode connected between a signal line and a ground line. Since the PIN diode has a variable resistance depending on the forward current flowing therethrough, it causes a variable attenuation of a DC component of the signal flowing through the signal line, depending on the AGC voltage.
The signal attenuator with the PIN diode is however disadvantageous in that the central frequency of the attenuation characteristics thereof tends to vary as the resistance of the PIN diode varies, resulting in a change in the central frequency of the tuner. However, the central frequency of the tuner should be prevented from varying because a change in the central frequency of the tuner would bring about a tracking error when a desired broadcast station is to be selected.